PROJECT SUMMARY Visual information travels along feedforward connections from the primary visual cortex (V1) through a hierarchy of cortical areas. In turn, the visual cortex receives a dense network of feedback (FB) connections from higher- order cortical areas. FB connections have been implicated in several forms of top-down influences, such as attention, expectation, and context dependent visual processing, but the specific contributions of FB connections to visual perception have remained mysterious. The goal of this proposal is to uncover how FB from the second visual cortical area (V2) to primary visual cortex (V1) affects visual perception and neural response properties of V1 neurons in primates. This goal will be achieved by targeting optogenetic inactivation to the axons of FB connections in awake behaving marmosets performing visual tasks. Candidate?s previous studies in anesthetized marmosets have shown that V2 FB affects receptive field (RF) size, surround suppression and response gain of V1 neurons. To test how these neural level effects manifest in perception, FB connections projecting from V2 to V1 will be optogenetically inactivated in awake marmoset monkeys performing visual tasks. The proposed project builds directly upon the candidate?s existing training in optogenetics, electrophysiology and psychophysics, but provides significant new training in using awake behaving marmosets. Advances in genome editing tools and success in transgene germline transmission in marmosets have positioned the marmoset as an important model system for neuroscience. Therefore, new training in awake behaving marmoset research will provide a strong foundation for transitioning to independence, and a skill set that will position the candidate ahead of the competition in the use of genetically engineered marmosets in neuroscience.